Fibrinogen, the main structural protein in the blood responsible for the formation of clots, exists as a dimer of three polypeptide chains; the Aα (66.5 kD), Bβ (52 kD) and γ (46.5 kD) are linked through 29 disulphide bonds. The addition of asparagine-linked carbohydrates to the Bβ and γ chains results in a molecule with a molecular weight of 340 kD. Fibrinogen has a trinodal structure, a central nodule, termed the E domain, contains the amino-termini of all 6 chains including the fibrinopeptides (Fp) whereas the two distal nodules termed D domains contain the carboxy-termini of the Aα, Bβ and γ chains. Fibrinogen is proteolytically cleaved at the amino terminus of the Aα and Bβ chains releasing fibrinopeptides A and B (FpA & FpB) and converted to fibrin monomer by thrombin, a serine protease that is converted from its inactive form by Factor Xa. The resultant fibrin monomers non-covalently assemble into protofibrils by DE contacts on neighboring fibrin molecules.